1. Field of the Invention
The present invention relates to an optical transmission circuit which includes a semiconductor laser used as a light emitting element and controls the semiconductor laser so as to have a predetermined pulse width and a predetermined optical output power so that it is possible to realize a miniaturized and low cost circuit.
2. Description of the Related Art
Recently, optical fiber cables have been widely provided for subscribers in order to realize a multimedia communications system. In this system, an optical transmission/reception circuit is provided in each subscriber""s terminal. In general, an optical transmission circuit for the subscriber is driven in such a way that a bias current of the semiconductor laser is set to zero in order to realize a large extinction ratio Pp/Pb, (where, Pp is an output intensity at a top of a pulse, and Pb is an output intensity at a bottom of the pulse).
In this case, however, when the bias current is set to zero, it is difficult to obtain a desirable optical pulse width due to a delay of oscillation of the semiconductor laser. In general, the delay Td of oscillation can be expressed by the following formula.
Td=xcfx84sxc2x7log(Ip)/(Ip+Ibxe2x88x92Ith)xe2x80x83xe2x80x83(1)
Where, xcfx84S is a carrier life time, Ip is a modulation current, Ib is a bias current, and Ith is a threshold current.
As is obvious from the above formula (1), when the bias current Ib is increased, it is possible to reduce the delay Td of oscillation. In this case, however, since the optical transmission circuit for the subscriber is driven in such a manner that the bias current of the semiconductor laser is set to a small value, i.e., approximately zero, in order to realize low power consumption and the large extinction ratio, there is a problem in which the delay Td of oscillation becomes large as is obvious from the above formula (1).
In order to resolve the above problem, i.e., in order to compensate for the above delay of oscillation, there is a known structure which adjusts a duty value of data supplied to a drive circuit of the semiconductor laser. There are, however, further problems, i.e., complicated adjusting processes due to use of two adjusting circuits for performing duty adjustment and optical power adjustment, as explained in detail below.
The object of the present invention is to provide an optical transmission circuit which can realize the duty adjustment and the optical power adjustment by using only one adjusting circuit so that it is possible to achieve a simplified, miniatuarized, and low cost optical transmission circuit.
In accordance with the present invention, there is provided an optical transmission circuit including: a semiconductor laser; a drive circuit connected to the semiconductor laser for supplying drive current to the semiconductor laser; a duty changing circuit connected to the drive circuit for receiving an input data and changing a duty value of the input data; a converting circuit connected to the duty changing circuit for supplying a duty control signal to the duty changing circuit in order to adjust the duty value of the input data; and an adjusting circuit connected to the converting circuit and the drive circuit for supplying an optical power control signal to these circuits.
In a preferred embodiment, the converting circuit converts the optical power control signal, which is supplied from the adjusting circuit to the converting circuit and the drive circuit, to the duty control signal in order to control the optical power of the semiconductor laser in such a manner that when the optical power is increased, the duty value is decreased.
In another preferred embodiment, the converting circuit comprises an analog-to-digital (A/D) converter which converts the optical power control signal to a digital signal used as an address signal; a memory accessed by the address signal from the A/D converter and for storing the duty control signal in correspondence with the address signal; and a digital-to-analog (D/A) converter which converts a digital duty control signal read out from the memory to an analog duty control signal in order to supply the analog duty control signal to the duty changing circuit.
In still another preferred embodiment, the converting circuit comprises an analog-to-digital (A/D) converter which converts the optical power control signal to a digital signal used as an address signal; a memory accessed by the address signal from the A/D converter and for storing the duty control signal in correspondence with the address signal; and a digital-to-analog (D/A) converter which converts a digital duty control signal read out from the memory to an analog duty control signal in order to supply the analog duty control signal to the duty changing circuit.
In still another preferred embodiment, the optical transmission circuit further comprises a temperature compensating circuit which is provided between the adjusting circuit and the drive circuit in order to compensate for a temperature characteristic of the semiconductor laser.
In still another preferred embodiment, the optical transmission circuit further comprises a temperature compensating circuit which is provided between the adjusting circuit and the duty changing circuit in order to compensate for both the temperature characteristics of the semiconductor laser and the duty changing circuit.
In still another preferred embodiment, the temperature compensating circuit comprises a temperature detecting element, an analog-to-digital (A/D) converter for converting a detection signal detected by the temperature detecting element to a digital signal used as an address signal, and a memory for storing temperature data accessed by the address signal from the A/D converter; and wherein the optical transmission circuit further comprises an analog-to-digital converter (A/D) connected to the adjusting circuit, a multiplier connected to the A/D converter and a digital-to-analog converter (D/A) connected to the multiplier; the A/D converter converts an output of the adjusting circuit to a digital signal; the multiplier multiplys the digital signal by an output of the memory in the temperature compensating circuit; and the D/A converter converts an output of the multiplier to an analog signal in order to obtain the optical power control signal to be supplied to the drive circuit.
In still another preferred embodiment, the optical transmission circuit further comprises a photodiode for monitoring the optical power of the semiconductor laser; and an automatic power control (APC) circuit provided between the photodiode and the drive circuit for detecting current which flows in the photodiode, and to provide the optical power control signal to the drive circuit based on the detected current; wherein the adjusting circuit is connected between the photodiode and the APC circuit in order to perform the initial setting of the current which flows in the photodiode; and the converting circuit converts an output of the adjusting circuit to the duty control signal.
In still another preferred embodiment, the converting circuit comprises an initial value setting unit; a voltage/current converter for converting the current, which flows based on the optical power control signal from the adjusting circuit, to a voltage; and a linkage adjusting unit for outputting the duty control signal based on the initial setting value from the initial value setting unit in correspondence with an output voltage from the voltage/current converter.
In still another preferred embodiment, the converting circuit comprises an initial value setting unit; a voltage/current converter for converting the current, which flows based on the optical power control signal from the adjusting circuit, to a voltage; a power fluctuation compensating unit for detecting fluctuation of power voltage and correcting the output voltage from the voltage/current converter; a voltage/current converter for converting the output voltage of the power fluctuation compensating unit to a current; and a linkage adjusting unit for outputting the duty control signal based on the initial setting value in correspondence with an output voltage of the voltage/current converter.
In still another preferred embodiment, the converting circuit comprises an initial value setting unit; a voltage/current converter for converting the current, which flows based on the optical power control signal from the adjusting circuit, to a voltage; a resistance value control unit for converting the output voltage of the voltage/current converter to a resistance value control signal; and a linkage adjusting unit for outputting the duty control signal based on the initial setting value in correspondence with the resistance value control signal.
In still another preferred embodiment, the duty changing circuit is formed by a clock reference type duty changing circuit which receives input data and clock signals, and adjusts the duty value of the input data to be supplied to the drive circuit in accordance with the duty control signal from the converting circuit.